In a crankcase scavenged engine with intake and exhaust porting, the timings of the port events are symmetrical, with the exhaust opening first and closing last. There is reason to believe that the blowdown process occurring when the exhaust port first opens is richer in unburned or partially burned fuel. During exhaust port closing fresh charge is lost to the exhaust reducing the average temperature of the exhaust, the effective compression ratio and the fraction of fresh charge trapped compared to total charge supplied. It is the intent of this invention to improve at least some of these aspects of conventional porting.